A group-III nitride semiconductor light-emitting device that emits light in the blue or green band is composed of a multilayer structure where a gallium nitride (GaN) crystal layer as one constituent element is disposed, for example, on a sapphire (α-Al2O3) single crystal substrate using a growing technique, such as metal organic chemical vapor deposition (MOCVD) method. The multilayer structure has a light-emitting part structure undertaking the function of emitting light. Conventionally, the light-emitting part structure in general takes a pn junction-type hetero structure constructed by a p-type or n-type clad layer consisting of a light-emitting layer formed of gallium indium nitride (GaYIn1−YN, wherein 0<Y≦1) and an aluminum gallium indium nitride (AlGaInN)-based crystal layer.
FIG. 5 is a schematic sectional view showing an example of the construction of a conventional multilayer structure light-emitting device (LED) 100 having a pn junction-type double hetero (DH) junction light-emitting part structure 42 comprising an AlGaInN-base crystal layer. In a conventional multilayer structure, the light-emitting part structure 42 is composed of, for example, a lower clad layer 103 comprising an n-type aluminum gallium nitride (AlZGa1−ZN, wherein 0≦Z≦1) crystal layer, a light-emitting layer 104 comprising an n-type gallium indium nitride (GaYIn1−YN) and an upper clad layer 105 comprising a p-type aluminum gallium nitride (AlZGa1−ZN, wherein 0≦Z≦1) (see, JP-A-6-260682 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). The functional layers 103 to 105 constituting the light-emitting part structure 42 each is usually deposited with an intervention of a buffer layer formed at a temperature lower than the temperature at the formation of those functional layers, so-called a low-temperature buffer layer 102 (see, JP-A-4-297023). Furthermore, in a multilayer structure comprising a sapphire substrate 101 having; provided thereon a group-III nitride semiconductor crystal layer, the low-temperature buffer layer 102 is usually composed of aluminum gallium nitride (AlZGa1−ZN, wherein 0≦Z≦1) (see, JP-A-6-151962).
The low-temperature buffer layer 102 is provided mainly for the purpose of reducing the lattice mismatch between the sapphire substrate 101 and the lower clad layer 103 composed of AlZGa1−ZN crystal, thereby obtaining a good group-III nitride single crystal layer reduced in the density of crystal defects such as dislocation. Particularly, in a known conventional example, the low-temperature buffer layer 102 is composed of gallium nitride (GaN), the lower clad layer 103 is composed of a GaN layer formed at a high temperature in excess of the temperature for the formation of the low-temperature buffer layer 102, and the light-emitting layer 104 is composed of a gallium indium nitride mixed crystal phase (see, JP-A-6-216409).
Furthermore, in the conventional light-emitting device shown in FIG. 5, the substrate 101 is an insulating sapphire and therefore, a part of the lower clad layer 103 must be removed to provide an n-type ohmic electrode 107. A p-type ohmic electrode 106 is provided on the electrically conducting upper clad layer 105.
However, the mismatch between the sapphire substrate and the GaN layer constituting the low-temperature buffer layer is as high as about 13.8% (see, Nippon Kessho Seicho Gakkai Shi (Journal of Japan Crystal Growth Society), Vol. 15, Nos. 3 & 4, pp. 74–82 (Jan. 25, 1989)) and therefore, a continuous low-temperature buffer layer cannot be stably obtained at present. In the discontinuous portion partially present in the low-temperature buffer layer due to the lacking of film continuity, namely, in the region where the sapphire substrate surface is exposed, hexagonal GaN predominantly grows in the c-axis direction thereof. As a result, dislocation is generated starting from the coalescence of GaN columnar crystals, propagates to the GaInN light-emitting layer via the upper GaN layer and disadvantageously deteriorates the crystal quality of the light-emitting layer. In other words, according to the above-described conventional multilayer structure constructed by stacking a GaInN light-emitting layer on a GaN low-temperature buffer layer via a GaN layer, a good GaInN-type light-emitting layer film cannot be formed due to the propagation of crystal defects such as dislocation attributable to the discontinuity of the low-temperature buffer layer. Therefore, stable formation of a light-emitting part structure comprising a group-III nitride single crystal layer having excellent operation reliability or ensuring long device life cannot be attained particularly in the case of a laser diode (LD).
The present invention has been made by taking into account these problems in conventional techniques, and an object of the present invention is to provide a buffer layer having continuity capable of allowing homogeneous coating on the substrate surface and preventing generation of dislocations despite a large mismatch with the substrate crystal. Another object of the present invention includes providing a construction of a group-III nitride single crystal layer structure deposited on the above-described buffer layer, which is reduced in the density of crystal defects such as dislocation and favored with excellent crystallinity.